1. Field of the Invention
The present invention relates to an image forming apparatus such as a copy machine, a printer, or a facsimile machine which includes a fixing unit and a curl correcting device. The fixing unit uses heat and pressure to fix a toner image on a sheet passing through nip portion formed between the fixing device and a pressing device. The curl correcting device corrects curl of the sheet formed after fixing.
2. Description of the Related Art
In general, in an electrophotographic process of the image forming apparatus, a heat-fixing process for heating a toner image transferred to a sheet is used. However, in the heat-fixing process, sheets are curled by heat, and the curl of the sheet affects sheet feeding and the stacking of the sheets. A conventional image forming apparatus may use a curl correcting device which curls the sheet reversely against the curl of the sheet by increasing the rolling angle of a discharge roller.
For example, a conventional image forming apparatus may be provided with a discharging roller and two subsidiary rollers downstream of the heated fixing-roller which contacts the pressing roller. The curl of the sheet is corrected by cooperation of the discharging roller and the subsidiary rollers.
Japanese Patent Laid-Open Application No. 2004-139118 discloses a sheet fixing device with a curl correcting device which corrects curl made by the fixing portion. Specifically, a protruding portion which corrects curl is provided perpendicular to a straight line between nip formed by a pair of rollers of the fixing device and a nip formed by a pair of discharging rollers.
Additionally, another known curl correcting device curls reversely against the curl of the sheet by changing the outer diameter or hardness of a discharging roller or discharging drum. However, in the fixing unit including the discharging roller with two subsidiary rollers as mentioned above, the amount of reverse curl which is needed varies depending on the type and size of the sheet, so adjustment of the rolling angle of the discharging roller is needed. However, when the pressure of the subsidiary rollers against the discharging roller is increased, the image quality of imprinted sheet is decreased. Additionally, for the fixing unit including discharging roller with two subsidiary rollers as mentioned above, since the two subsidiary rollers pressed against the discharging roller to correct the curl of the sheet and a spring are needed, the cost of the fixing unit increases.